It is relatively simple to assemble the valve-operating mechanism in an engine with the aforesaid bathtub-type combustion chamber. However, the combustion efficiency of this chamber is inferior to that of the aforesaid pent roof-type. In recent years, this has led to greater use of pent roof chambers.
FIGS. 18 and 19 show an example of an air-cooled single-cylinder overhead-valve four-cycle internal combustion engine with the aforesaid pent roof combustion chamber which belongs to the prior art. FIG. 18 is a cross section of the engine which includes the cylinder and the push rods. FIG. 19 is a cross section taken along line Z--Z in FIG. 18.
In FIGS. 18 and 19, 1 is the combustion chamber; 2 is the air-cooled cylinder; 5 is the crankshaft; 6 is the connecting rod; 7 is the piston; 8 is the cylinder head; 14a is the intake valve; and 14b is the exhaust valve (Hereafter, the aforesaid intake valve 14a and exhaust valve 14b will be referred to in common as induction/exhaust valves 14.)
17 is the camshaft, which is engaged with the aforesaid crankshaft 5 through a gear train; 17a is the cam on the said camshaft 17; 16 is the tappet; 15 is the push rod; 13 is the rocker arm shaft, which is fixed to and supported on rocker arm supporting base 22, which is itself fixed to the top of the aforesaid cylinder head 8. 11 is the rocker arm, which engages with the said rocker arm shaft 13 in such a way that it is free to swing. 18 is the valve spring. 19 is the valve spring bearing. 9 is the head cover, which is mounted on top surface 8b on top of cylinder head 8 and which covers the mechanism which operates the valves. When this engine operates, induction/exhaust valves 14 open and close according to a timing determined by cam 17a, whose rotating speed is reduced to half that of crankshaft 5 by a timing gear (not pictured).
In FIG. 18, the rotation of camshaft 17a forces push rods 15 upward, and rocker arms 11 swing around shaft 13. Intake valve 14a or exhaust valve 14b is pushed upward against the elastic force of valve spring 18, and the valve opens.
In an OHV engine like this, to insure that the action of cam 17a is transmitted reliably to induction/exhaust valve 14 through push rods 15, the aforesaid valve spring 18 must have a relatively large spring constant, meaning that a strong spring must be used; and rocker arm shaft 13 must have a relatively large diameter.
To insure that the contacting surfaces of the valve operating mechanism do not experience excessive force when the engine is running and the cylinder head gets hot, an adjustment screw (not pictured) is provided to adjust the clearance between the contacting portions of rocker arms 11 and push rods 15.
In the aforesaid cylinder head 8, the aforesaid head cover 9 is hermetically sealed to top surface 8b, the upper surface of peripheral wall 8c, which surrounds the head. The aforesaid rocker arm supporting base 22 for the rocker arms is bolted to an area in the center of upper surface 8a which is lower than the said top surface 8b by a fixed amount.
In the four-cycle overhead valve internal combustion engine from the prior art which is pictured in FIGS. 18 and 19, there are two surfaces at the top of cylinder head 8, 8b and 8a. 8b is the top surface onto which head cover 9 is fixed; 8a is the mounting surface on which rocker arm supporting base 22, which supports the rocker arms, is fixed. These two surfaces must be finished by a machining process so that they are relatively smooth.
However, in the prior art cylinder head 8, top surface 8b, on which cover 9 is mounted, and mounting surface 8a, on which rocker arm supporting base 22 is mounted, are at different heights. This means that they must be machined in a two-stage process or that the machinist must change tools in mid-process. This increases the number of processes required and incurs an extra cost for set-up.
Designs for overhead valve engines with a hemispherical combustion chamber and the intake and exhaust valves arranged so that they radiate from the center have been proposed in Japanese Patent Publications (Kokai) Hei5-133205. In this prior art, one intake valve, one exhaust valve, and one spark plug are arranged so that the angles of these center lines (L1), (L2), (L3) against the center line of cylinder are same as each other, and they are located at a same distance from the center of the cylinder in order to manufacture the cylinder easily.
Another prior art is proposed in Japanese Patent Publications (Kokai) Hei5-133205. In both of these, however, the structure which supports the valve operating mechanism in the cylinder head is three-dimensional. It is difficult to achieve the high level of precision required by the processing, and the structural components of the valve operating mechanism experience torsion force when the valves are driven, which shortens their service life.
For a structure of a lubrication device for an OHV engine, there is a breather passage between the crankcase and the valve operating mechanism chamber which contains the valve operating mechanism. Oil which is taken up by a dipper, splashed about and suspended in the crankcase is conveyed via this breather passage into the aforesaid valve operating mechanism chamber with the movement of air caused by the downward stroke of the piston. In this way the said valve operating mechanism chamber is lubricated.
An example of an existing lubrication device for the valve operating mechanism in a small multipurpose OHV engine can be found in Japanese Utility Model Publication (Kokoku) 63-15530. The details of this device are shown in FIGS. 20 through 22.
These drawings show an OHV engine whose cylinder is canted upward from the horizontal. Breather passage 131, which connects crankcase 101 and valve operating mechanism chamber 102, is formed within the walls of cylinder barrel 116 and cylinder head 118.
The end portion 131a of the said breather passage 131 in valve operating mechanism chamber 102 faces from above intake valve 151 toward the point where valve stem 152a of exhaust valve 152 and rocker arm 162 come in contact. Branching passage 131b faces to the point where valve stem 151a of intake valve 151 and rocker arm 161 come in contact.
Because this OHV engine is configured in this prior art, the air which is moved by the downward stroke of piston 107 forces the oil picked up by dipper 115 and suspended in crankcase 101 into the aforesaid breather passage 131. The greater part of this suspended oil goes in a straight line through portion 131a and is splashed upon the operating mechanism for exhaust valve 152 in the vicinity of the point where valve stem 152a and rocker arm 162 come in contact. This is how most of the suspended oil is supplied.
The remainder of the suspended oil goes through branching passage 131b and is splashed upon the operating mechanism for intake valve 151 in the vicinity of the point where valve stem 151a and rocker arm 161 come in contact.
When the air forced into the aforesaid valve operating mechanism chamber 102 goes through breather valve 108, the lubricating oil is separated out. The air enters breather chamber 109, travels through breather tube 132 and is returned to carburetor 111. The oil flows down the interior surface of valve operating mechanism chamber 102. It goes through the space around push rod 122 and tappet 121 and is recovered in crankcase 101.
In this prior art OHV engine disclosed in the Japanese Utility Model Publication (Kokoku) 63-15530, as may be seen in FIG. 21, intake and exhaust valves 151 and 152 are parallel to each other, and the distance traveled by the aforesaid two valves, which protrude into valve operating mechanism chamber 102, is relatively short. Breather passage 131, which goes through the aforesaid crankcase 101 and valve operating mechanism chamber 102, is formed in the thick portion within the walls of cylinder barrel 117 and cylinder head 118.
In recent years, more and more pent roof combustion chambers have been used in OHV engines to increase combustion efficiency. In an engine with a pent roof combustion chamber, the intake and exhaust valves are canted at a given angle with respect to the axis of the cylinder barrel, with the open side of the angle toward the exterior. As a result, a large space must be provided at the front end of the intake and exhaust valves, where they protrude into the valve operating mechanism chamber for the operating mechanism. At the same time, every possible structural component has been made thinner in the interest of reducing the weight of the engine, and every possible space has been made smaller. With the prior art design, it has proved impossible to simplify the breather passage without increasing the parts count. With the current breather passage, the exhaust valve does not receive sufficient lubrication, which shortens the service life of the engine.